The present invention relates to a method of making silicon nitride-bonded silicon carbide honeycomb bodies useful in the removal of diesel soot particles from the exhaust gas of diesel engines.
Filters of diesel exhaust streams require a combination of high thermal shock resistance, chemical and mechanical durability in harsh environments, and good filtration efficiency.
Cordierite monolith filters have been in use for heavy duty engine applications for nearly two decades. Recently, there has been an increase in demand of filters for passenger diesel cars due to tightening environmental pollution regulations. However, it appears that in passenger cars, especially under conditions of xe2x80x9cuncontrolledxe2x80x9d regeneration (i.e., where the onset of combustion coincides with, or is immediately followed by high oxygen content and low exhaust gas flow rates resulting in high temperature spikes) existing cordierite filters have shown tendency for failure especially due to cracking or melting.
As an alternative to cordierite monolith filters, silicon carbide (SiC) filters have emerged as an option for diesel engine automobile applications. Benefits of these filters reside in the SiC""s stable composition, high mechanical strength and high thermal conductivity. However, SiC has poor thermal shock resistance that results in cracking during regeneration. Consequently, current SiC diesel particulate filters have to be designed in segmental configuration to minimize cracking during use, which translates into higher manufacturing and product costs.
There is, accordingly a clear need for a process for making a monolith silicon carbide-based honeycomb body for diesel exhaust filtration applications.
Accordingly, the present invention is directed at a method of forming a silicon carbide-based honeycomb filter body by using silicon nitride to generate a uniform, strong bonded silicon carbide structure capable of being formed into a monolith honeycomb body having good mechanical strength, uniform pore structure and desirable thermal properties.
Specifically, the invention is directed at a process for forming a silicon nitride-bonded silicon carbide composite honeycomb monolith by a) forming a plasticizable mixture which includes (1) about 60% to 85% by weight, powdered silicon carbide; (2) about 10% to 40% by weight, powdered silicon metal; and, (3) organic components; b) extruding the plasticizable mixture to form a green honeycomb monolith; c) drying the green honeycomb monolith in air; and, d) heating the honeycomb monolith to 1450xc2x0 C. with a hold of 1 hour in an atmosphere of argon; and, e) nitriding the honeycomb monolith above 1450xc2x0 C. up to 1600xc2x0 C. for a time sufficient to obtain a silicon nitride-bonded silicon carbide body. The gas environment is provided either by flow of the gas at a rate from 2 cubic feet per minute (cfm) or holding at 2 to 6 pounds per square inch (psi) positive pressure without continuing to flow the gas.